Line a half sheet pan with a nonstick rubber mat. Place a stainless steel frame 25cm / 10in by 38cm / 15in by 3.5cm / 1.5in on top of the rubber mat.
Bring the milk and cream to a boil with the tea leaves. Let it steep for 5 minutes off the heat and strain the mixture through a fine-mesh strainer.
Bring the infused liquid up to a rolling boil again and pour over the chocolate. Let it sit for 45 seconds. Stir with a rubber spatula until the chocolate has melted. Try to not stir too hard or to stir air into the ganache (to prevent bubble formation).
Add the butter and stir until it has melted.
Pour the contents into the prepared sheet pan and spread evenly. Refrigerate.
Once hardened, trim off the borders and cut into 3.75-cm / 1.5-in squares. Dust the top with cocoa powder, reserve refrigerated. Discard after 5 days.